I've owned at least a dozen 303s in the last 30 years. Last time i had one i compared it to some of the more recent kit we've built (it was very disappointing). Unless you are just going to use it as the basis of something else, i'd just make it work as stock, and flip it.

Their is some thing lovely about the Quad 303. The price and styling are without comparison on UK ebay for power amplifiers. They sound so much better than the Quad 405. I would always recommend replacing all capacitors in a Quad 303 as they are very old (I would recommend doing this in all audio kit over 20 years old and the 303's are older than that).

I have 3 Quad 303's and refurbishing them makes them sound a million times better than they sound without new Capacitors. Sadly they are not good at three things:

Bass response is poor when comparing to a Quad 405 and Quad 306 (capacitor coupling).

Handling of low impedance loads is not great.

Bi-amping/active cross overs.

Quad 303's wont break with hard to drive loads of low impedance as they are intended to drive Quad ESL 57's which have an 2 Ohms impedance but this is in the high frequencies so the powers are low and apart from the JLH class A amp the beat all the limited number of amps that I have heard driving Quad ESL's.

Another problem with the Quad 303 for bi-amping or connecting directly to a computer is that they regulate the Earth rather than the positive rail which means you need to avoid shorting the 303's Earth with your other amps earth (I killed the power regulators two or three times before I realized, I should have looked at the circuit), I would suggest using a single ended to balanced driver in front of each Quad 303 and if using just one and a computer definitely use an optical connected DAC if your using a desktop computer. (laptops don't define the earth so it wont matter so much but is still a good idea)

When I have finished a few more projects, (specifically my Active cross over and rebuilding my Quad 306's) I want to compare my Quad 303 power amps with my Quad 306 (after capacitor replacement to give them a chance) because my tests currently favor my Quad 303 for sound (excepting bass compromises) but Quad 306's are being used due to the annoying earthing issue I mentioned earlier. The Quad 306 is a lot better than the Quad 405 even before replacing the old capacitors in my 306.

In terms of treble sound, I think Quad 303's are very nice, though a JLH 15 watt class A amp that I built (but never had big enough transformer so it started to hum) did sound better and moving from the 1969 to 1996 circuits showed to me that the capacitor coupling in the Quad 303 output is responsible for the poor bass (even with triple size compared to the circuit from Quad) as this was equal to the 1969 amplifier topology (using a coupling capacitor) and when I moved to the 1996 topology (no output coupling capacitor) the Bass in the JLH outperformed the Quad 303.

I hope this does not lead you to strip out the electrics of the Quad 303 and use it as a gain clone as a Quad 303 is a classic amplifier well worth restoring, but a well built JLH class A amplifier will sound nicer but is a lot more work.